The reason they don't is simple. They don't have enough satellite bandwidth to do that with the "spot beams" they use to deliver local stations to their individual markets, and if the diginets made a deal with DirecTV to carry them as a single national feed it would probably violate the terms of their carriage agreements with the local stations in each market.
And you won't even notice, because the PSIP information will not change, regardless of which transmitter a station is using.
Let's say channels 4.1, 4.2 and 4.3 are actually transmitting from channel 48, and in the repacking they start using channel 36's transmitter, which already has channels 7.1 and 7.2 on it. 4.1 would become the third stream on 36, but it would still be displaying as 4.1 via PSIP.
You might have to rescan after packing, but that's about it.